Friday, June 4, 2010

When nature calls

Imagine the scene where you have been exercising hard in the heat for a long period and have not taken the time to drink fluids, not unusual for a kayaker. You’ve been sweating and your body is dehydrated. You finally take the time to drink some water. To quench your thirst, you finish off an entire water bottle within seconds. Feeling refreshed, you continue paddling. Within minutes, you get the annoying urge to urinate.

To rehydrate, it is in the athlete’s best interest to replace 100% of fluids lost. The amount of ingested fluid that is retained (not converted to urine) reflects the efficiency of rehydration. Thus, the athlete would want 100% efficiency. So what affects efficiency? A recent study by E. J. Jones and colleagues from Stephen F. Austin State University and the University of Alabama tested the effects of a large amount of water intake vs smaller amounts taken intermittently over a period of time on rehydration efficiency.

In their study, 8 healthy men walked on a treadmill in an environmental temperature of 95ºF (35ºC) until they achieved a 2% loss of body weight. For a 170 lb person, 2% is equivalent to 3.4 lb or 1.6 liters of water. Following the exercise, participants were given a light breakfast and 4 hrs later were fed lunch. Each participant performed this protocol twice; one time given a large amount of water during breakfast equivalent to the amount lost during exercise (1.6 liters) and another time the same quantity of water given in 8 smaller doses over a 4-hr period (0.2 liters every 30 min). Urine was collected and measured over an 8-hr period following exercise. Hydration efficiency was determined as the (fluid lost - urine produced/fluid lost x 100).

What they found

  • Urine production following one dose of water equivalent to amount lost was .70 liters vs .42 liters when same amount of water was given in 8 smaller doses over 4 hrs
  • Hydration efficiency (the actual amount ingested that stayed in the body) was 55% following one dose of water equivalent to amount lost vs 75% when same amount was given in smaller doses
  • Within the 8-hr period, about 2/3 of the urine produced occurred during the first 4 hr regardless of the method of water consumption

What this means

Aside from the results of this study, it has been previously shown that in order to 100% rehydrate with water, 150% of what was lost must be consumed. One explanation for this is that the kidneys are over stimulated in an attempt to maintain normal blood osmolality (electrolyte concentrations) and volume. This is because water consumption reduces blood osmolality quickly. The response is a decreased secretion of the anti-diuretic hormone which leads to increased urine production. What is also known is that rehydration efficiency is higher when fluids containing electrolytes and carbohydrate (i.e. Gatorade) are consumed. Unlike plain water, sport drinks do not cause a rapid decline in blood osmolality, and thus, result in less urine production.

The bottomline

The overstimulation of kidneys can be more effectively avoided when water is consumed in smaller amounts over a period of time rather than consumed as a large amount at once. However, you will produce urine no matter what and you will need to consume additional water to compensate for this loss if you want to completely rehydrate. Drink fluids frequently during exercise and following exercise. Of course, if you are exercising and sweating for a period of time, you’ll need to replace carbohydrates and electrolytes, in addition to water. Most sport drinks do a good job replacing all three. Try to consume all three essentials during exercise as well as following exercise.

Reference: Jones et al, Effects of metered versus bolus water consumption on urine production and rehydration. Int. J. Sport Nutr Exerc Metab, 20, 2010.